A glow discharge starter is usually connected across or in parallel with an arc discharge lamp and contains a pair of electrodes. At least one of the electrodes comprises a bimetallic element which, when heated as a result of the glow discharge, bends towards the other electrode. When contact is made, the glow discharge ceases causing the bimetallic element to cool and withdraw from the contacted electrode. When contact is broken, a voltage pulse induced by the induction of the ballast, appears across the opposed electrodes of the lamp thereby initiating an arc discharge within the lamp. If the lamp ignition does not occur after the first voltage pulse, the glow discharge sequence is repeated until lamp ignition occurs.
Various glow discharge starters and other thermal devices are manufactured by first sealing a glass mount to a piece of glass tubing (i.e., envelope) by means of a press seal. The mount supports the electrodes of the device. Often during the manufacturing process, the press sealing operation causes the axis of the electrodes to be angled (i.e., misaligned) with respect to the tubing. If this misalignment is great enough, either the bimetallic electrode or the counter electrode touch the internal surface of the glass tubing. Consequently, the electrical characteristics of the glow discharge starter are altered.
The heat and gases from the sealing fires used during the press sealing operation flow upwards through the glass tubing between the stem of the mount and the internal surface of the glass tubing in a so-called "chimney effect". As a result, the surface of the bimetallic element is unwantonly oxidized.
The lower portion of the glass stem which is sealed to the glass tubing is relatively thick which often prevents this section of the stem from reaching the proper sealing temperature. Consequently, thermal cracks often appear in the region of the press seal. Increasing the sealing fires in an effort to adequately heat the lower portion of the stem has a two-fold effect. First, the increased heat is transferred upwards through the stem causing softening of the entire stem. At the same time, the sealing heat causes the bimetallic element to press against the counter electrode. Because of the softened stem, the bimetallic element causes a permanent distortion in the parallel relationship of the electrodes. Second, the increased heat further increases the amount of oxidation on the bimetallic element.